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Hack the Scosche FMT4 FM transmitter! V2.0 


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Hack the Scosche FMT4 FM 

transmitter! V2.0 

Written By: Chris 


Clamp (Squeeze clamp, c-clamp, duct 

Jeweler's screwdriver set (1) 

Liquid electrical tape (or other paintable 
insulator) (1) 

Multimeter (1) 

Soldering/desoldering tools (1) 


Various colors of small-gauge wire (I 
used 22g cat 5 wire) (1) 

A compact 12v -> 3v regulator (I built a 
variable-output power supply out of 
Radio Shack parts) (1) 

A cigarette lighter adapter (1) 
...or whatever you need to connect to 
your 12v power source. Radio Shack 
sells clamp-style cigarette lighter 
adapters, or you may have one your can 
cut up and repurpose. 


Do you want a cheap car FM transmitter with (unlike most of the products you can buy) 
great audio quality and great signal strength? You want to get yourself a Scosche FMT4 and 
hack it! 

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Hack the Scosche FMT4 FM transmitter! V2.0 

This is technically illegal in the US , but so is transporting dentures , so take it with a grain of 
salt, I guess. 

My suggestion for avoiding legal issues would be this: "Don't be a jerk". Use an unoccupied 
frequency, and use the minimum power that you need to be happy. This can mean 
shortening the antenna (use a quarter- or an eighth-wavelength instead of a full- or half- 
wave), or lowering the input power. 

A brief test led me to believe that transmit power increases as input power increases past 
2.4v, but I've never seen a schematic of this thing (or a datasheet for the chip). At some 
point I will try to make time to check reception at various points with various input voltages 
to validate this theory. (Did you ever wish you had an oscilloscope?) 

Step 1 — Hack the Scosche FMT4 FM transmitter! V2.0 

• Only if you want to add an antenna: 
Pop the front cover. Just pry it off; 
it's glued on with a removable (and 
reusable) adhesive. 

• Scrape the adhesive as needed 
to expose the four screws 
holding the inner plate on. 

• Remove the four screws and 
carefully pry up the inner plate. 
It is clipped to the rubber strain 
relief device on the 
audio/antenna wire. 

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Hack the Scosche FMT4 FM transmitter! V2.0 

Step 2 

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Hack the Scosche FMT4 FM transmitter! V2.0 

• This step is for antenna upgraders 
only. For the purposes of this step, 
the "top" of the unit is the side with 
the battery/antenna cable. 

• Cut an antenna wire to length. 
Cheat sheet: 88.1 MHz fullwave: 
128" half: 64" 1/4 wave: 32" 
107.9 MHz fullwave: 104" half: 
52" 1/4 wave: 26" 

• Bend the tine on the battery 
connector furthest away from 
the audio/antenna cable vertical. 
Pull it out from the other side. 

• From the battery compartment 
side, poke your antenna wire 
through the hole you just made. 
Pull about a foot through. Put a 
C-shaped curve in the end of it, 
and run it under the switch on 
the right side of the case so it 
comes out near the top by the 
audio cable. Pull a little bit out 
and strip off about 0.5-1 mm on 
the end. 

• See the three gold connectors 
on the top right of the PCB? The 
topmost one is the other side of 
the antenna connector. The 
antenna is surface-soldered to 
the other side of this post. We're 
going to leave it right there and 
add an antenna. 

• Secure the unit to your work 
surface with a clamp. Flux the 
gold connector (very 

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Hack the Scosche FMT4 FM transmitter! V2.0 

sparingly!) and the tiny wire 
tip. Establish a good 
mechanical connection (so 
the wire sits on the the gold 
contact without you holding it 
there). Give it a little dab of 
solder. Be very careful. If it 
takes more than half a 
second, you are in danger of 
frying stuff. 

• Check the connection for 
strength with a couple gentle 
tugs. Pull the wire back 
through to the bottom of the 
case. Make sure it's not in the 
way of the switch. Leave a 
little slack, and use the slack 
to make some strain relief 
where the wire enters the 
battery compartment. (Just a 
couple twists at that point 
should do fine.) 

• Antenna upgrade complete! 
Reassemble the case. 

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Hack the Scosche FMT4 FM transmitter! V2.0 

Step 3 

• Build a 2.4-3v DC power supply 
that will fit into the battery 

• I used a Radio Shack LM317T 
adjustable regulator (which lets 
me choose my input power, but 
makes it a lot harder to fit into 
the battery case). You also need 
about 230 ohms' worth of fixed 
resistor (I used 2 470-Q Radio 
Shack 271 1115's in parallel), a 
0.1 f cap, a 1 f cap, a 5k-Q 
pot, and some disused 22 gauge 
cat 5 wire. 

• Voltage note: I did try this with a 
fixed 3.3v regulator package, 
and while it does work, it's too 
much input voltage and you can 
hear the overdrive. 

• Voltage note #2: I am not 100% 
sure that increasing input 
voltage increases output signal 
strength, but it seemed like it did 
in the one highly unscientific test 
I did using my wife's car for a 
portable radio. :p "normal" input 
voltage is probably 1 - 3v for this 
unit (2.4 or 3v at first, then it 
drops off to as the batteries 

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Hack the Scosche FMT4 FM transmitter! V2.0 

Step 4 

• There are 4 connections to make in the power supply: 

• 1) The 0.1 f cap, the wiper on the potentiometer, the 1.0 f cap, ground wire to the 
PCB (make this about 2" long), and input power ground. Make this last wire at least as 
long as you want the power cord to your car (or other power source) to be! 

• 2) Pin 1 on the LM317T, the 0.1 f capacitor, and +12V from the car/power source. 
Make this wire at least as long as you want the power cord to your car (or other power 
source) to be! 

• 3) Pin 2 on the LM317T, the fixed resistor, the 1 .0 f capacitor, and +3.0v out (make 
this wire about 2" long). 

• Pin 3 on the LM317T, one of the non-wiper pins on the varistor, and the fixed resistor. 

• Make these four connections temporarily (good mechanical connections with no solder). 
Check it VERY CAREFULLY for shorts before going on to the next step. Even a slight 
jiggle could create a short if your mechanical connections aren't solid. You can use a 
breadboard for testing if you want something that you can plug in more safely. 

• Bear in mind that when you actually assemble it, it needs to fit into the FMT4 battery 
compartment, so there won't be any fancy PCB or breadboard. ;) 

Step 5 

• Connect the assembled power supply to a +10-20v power source (such as a car cigarette 
lighter), and put a multimeter on the output wires. Adjust the pot for +2.4V. Once it works 
well, take the input power away, work out how to mush the whole mess into the battery 
compartment, and solder all the joints. Apply liquid tape to prevent shorts. 

• Before soldering, ensure that you can get to your pot screw without creating any shorts. 
You may wish to adjust this after you have it all together. 

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Hack the Scosche FMT4 FM transmitter! V2.0 

Step 6 

• Solder the positive power supply 
output to the + battery terminal (I 
made a 2mm flat coil of wire and a 
big blob of solder). 

• Solder the negative power supply 
output to the - terminal (I wound the 
wire around the inside of the spring 
and tinned the resulting mess). 

• Deal with the spring on the 
negative side so it doesn't come 
into contact with anything 
unexpectedly- you could trim it, or 
drown it in liquid electrical tape. If 
you want to be tidy, you could cut it 
off completely and attach the 
negative wire the same way as the 
positive one (which would also give 
you a bit more room in the 

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Hack the Scosche FMT4 FM transmitter! V2.0 

Step 7 

• Finish assembly. 

• Put the free end of the antenna 
wire and the two free ends of the 
12V wires through the battery 
cover, and put the battery cover 
back on. 

• Put a power connector on the 
end of the power wires (I used a 
car cigarette lighter adapter from 
Radio Shack; you could do the 
same, or salvage one from an 
old piece of equipment.) 

Step 8 

• Success! Lower your input power 
and/or trim your antenna down as 
needed. This transmitter is a 
fraction of a watt, but with a 
fullwave antenna on it, I imagine 
you could annoy people a quarter 
mile away, so be a good neighbor. 

This document was last generated on 2012-11-01 12:19:18 AM. 

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